Abstract. This paper describes an application of a Lowry type model in a comparative study of the urbanisation effects, on an outer metropolitan sub-region, of locating the third London airport on two of the sites proposed by the Roskill Commission. The sites are at Cublington in Buckinghamshire and Thurleigh in Bedfordshire. No detailed attempt is made to explain the formal structure of the model. Attention is focussed instead on a description of the impact, in terms of activity change and inter-urban journeys, on the spatial structure of the Bedfordshire sub-region. The model is used first in a one-shot conditional prediction of the growth of the sub-region without an airport, and then including the airport in each of the proposed locations.